This invention relates to a liquid etchant used for etching of a copper foil or a copper plate and a method for roughening a copper surface, and more particularly to a liquid etchant for acicularly roughening a surface of copper and a method for etching a surface of copper to roughen the surface.
A multi-layer printed board is typically constructed by laminating an inner layer material, an outer layer material and prepregs on each other. In general, manufacturing of such a multi-layer printed board thus constructed is carried out by roughening a surface of a copper conductive pattern layer of the inner layer material and then laminating the outer layer material made of resin, a film, an ink or the like on the roughened conductive pattern of the inner layer material to prepare a laminate. Then, the laminate is formed with through-holes and then subjected to electro-plating.
Roughening of the copper conductive pattern layer is conventionally carried out according to any one of three methods. A first method is to form a layer of oxide such as cuprous oxide or cupric oxide on the surface of the copper conductive pattern layer. A second method is to reduce such an oxide layer to copper metal while keeping a shape of the oxide layer unchanged. A third method is to form a metallic copper layer increased in particle diameter on the surface of the copper conductive pattern layer by electroless copper plating.
Unfortunately, the first method, when copper oxide exposed on an inner surface of each of the through-holes is immersed or dipped in an acidic liquid such as a plating solution, causes the exposed copper oxide to be dissolved in the acidic liquid, leading to a defect called a pink ring. The second method requires to use an expensive reducing agent after formation of the oxide, to thereby cause not only an increase in number of steps in the manufacturing but an increase in manufacturing cost. Also, the third method likewise causes an increase in number of steps in the manufacturing.
In view of the foregoing, the assignee proposed etching techniques of subjecting a surface of copper to etching using an liquid etchant containing an oxo acid, an azole (in particular, benzotriazole) and a halide, as disclosed in Japanese Patent Application Laid-Open Publication No. 96088/1998. The proposed techniques permit a copper conductive pattern to be provided with a roughened surface increased in acid resistance while keeping the number of steps from being increased, to thereby solve the problems encountered by the first to third methods described above.
However, the proposed techniques necessarily require to combine benzotriazole with a halide. Also, the halide must be added in an amount of about 100 mg/l, to thereby stabilize a dissolving rate or etching rate of copper. This causes the etching rate to be reduced to a level as low as 0.5 pm/mm. Thus, the proposed techniques require a considerable period of time as long as about 4 to 6 minutes for formation of the roughened surface, leading to a failure in exhibiting increased productivity.
More particularly, the proposed techniques cause the etching rate with respect to a chlorine ion concentration to exhibit such characteristics as shown in FIG. 5. The etching rate is drastically increased or varied when the chlorine ion concentration is reduced to a level below 40 mg/l, so that it is highly difficult to keep the etching rate at a constant level. Thus, the proposed techniques require to adjust the chlorine ion concentration at a level of about 100 mg/l in order to stabilize the etching rate. Unfortunately, this leads to a reduction in the etching rate.